Long distance optical transmission systems generally require a plurality of amplifiers located along the length of the optical fibers to periodically amplify the optical signals. It is essential in these systems to provide the ability to remotely monitor the performance of any amplifier, and to locate the source of system degradation or fault to a particular amplifier or fiber section.
Most known methods for remotely monitoring the performance of optical amplifiers in an optical transmission system require an optical loopback path between adjacent amplifiers on the forward and return optical paths, and the generation of a test signal on at least one end of the system. For example, U.S. Pat. No. 5,436,746 discloses an optical transmission system that includes multiple loopbacks. A test signal is generated at the local station, or terminal, and transmitted on a forward path. The test signal is returned to the local station via the optical loopbacks and a return path. Measurement of the test signal provides information that is related to the performance of the amplifiers within the optical transmission system.
The method of using loopback paths to remotely measure the performance of amplifiers has several disadvantages. Specifically, the loopback method requires test signals to be both transmitted and received on associated fiber pairs at a terminal of the transmission system. The test signals must travel over an optical fiber pair (i.e., the forward path and the return path). Therefore, the loop loss information provided by the loopback method is ambiguous because there is no way to tell how the loop losses are distributed between the forward and return path.
Further, the loop loss information provided by the loopback method is redundant because the same information is measured at both terminals of the transmission system. In addition, the optical loopback paths between adjacent amplifiers cause a significant transmission impairment in the form of crosstalk or added noise. Finally, the loopback method, when used to provide information in-service (i.e., while the optical transmission system is transmitting signals) requires a long time (approximately 2-8 hours) to obtain a measurement due to the typical poor signal-to-noise (S/N) ratio of the monitoring signal. Transmission systems that utilize multiple carrier wavelengths, and their corresponding monitoring signal, have lower S/N ratios than single wavelength systems, and therefore obtaining measurements using the loopback method in these systems impose an even greater time delay.
Based on the foregoing, there is a need for a method and apparatus for remotely measuring amplifier performance that provides measurement information quicker and more accurately than known methods, especially when multiple carrier wavelengths are used.